Bending processes of this kind are known in various forms, both in the bending of sheets of glass in a vertical position and in the bending of sheets of glass in a horizontal position. In the vertical techniques, the sheets of glass are as a rule carried into the bending press on nippers. When bending the sheets of glass in a horizontal position, the glass sheets are generally heated to bending temperature in a roller type continuous furnace and brought into the bending press in a horizontal position.
The manufacture of thermally prestressed glass sheets with curves of varying degrees of sharpness, for example the manufacture of glass sheets with curved ends, as for instance used for car rear windows, is a critical process, particularly when the glass sheets are heated to bending temperature in a roller type furnace in a horizontal position. In such cases, more frequently than elsewhere, the curved glass sheets break during the abrupt cooling that follows the bending process for the purpose of thermal prestressing.
The increased risk of breakage is attributable to the increased risk of distortion to which the glass sheets are subject under the effect of their own weight when transported horizontally, for the following reason. To counter the risk of distortion, the glass sheets are heated to a temperature at only the lower limit of the temperature range required for bending and subsequent prestressing. However, the lower the temperature at the moment of bending, the greater the risk that glass sheets to be sharply curved will fail to withstand the bending process itself or the subsequent prestressing process and will break.